The present invention relates to medical diagnostic systems in general and specifically to non-invasive measurements of intra-ocular pressure.
Glaucoma is a condition of optic nerve degeneration resulting in loss of vision, characterized by an elevation in intra-ocular pressure (IOP). If left untreated, severe loss of vision and eventual blindness may occur. Several methods of treatment, including medication, laser and surgical procedures are employed in combating this condition. The common goal of all the methods is to reduce the IOP so that vision loss is kept to a minimum.
In controlling glaucoma, it is necessary to closely monitor the IOP. This is generally accomplished in a physician""s office, hospital or clinic at periodic visits. Several diagnostic systems are available for this purpose.
All of the diagnostic systems in use today are versions of a mechanical tonometer, which are based on direct pressure measurement, as shown in FIGS. 1A and 1B, to which reference is now made. FIG. 1A is a depiction of an eye 20 with a cornea 22 and lens 24. FIG. 1B is a depiction of eye 20 whose cornea 22 is flattened by a force F.
Defining pressure P as force F per area A (P=F/A), IOP is obtained, as shown in FIG. 1B, by applying force F to a planar surface 26 (in this case, a circular surface having a diameter D) against cornea 22 and measuring force F required to flatten cornea 22 to an area measured on the planar surface. Pressure P is then calculated as the ratio of force F to the determined area.
The general methods of measuring IOP are: a) to apply a force and measure the resulting flat area (either directly or indirectly by measurement of deflection and calculation of resulting area based on an assumed corneal radius) or b) to depress the cornea to attain a given diameter flat area, measure the required force and calculate the resulting pressure as force per unit area.
The most widely used mechanical tonometer is the Goldmann Applanation Tonometer, described in U.S. Pat. No. 3,070,997. The general to principle is that shown in FIG. 1B, of direct pressure measurement following applanation (or flattening) of cornea 22. A flattened diameter of 3.06-mm has been determined by experimental methods to be optimal for this method. The flat contact surface is chosen so that no component of corneal tension is perpendicular to the cornea tonometer interface. This method is considered the xe2x80x9cGold Standardxe2x80x9d although it suffers inaccuracies due to variations in measurement caused by differences in corneal thickness and the angle of application of force F.
Other tonometers have been developed using the same principle of measuring force per flattened/indented area. Indentation tonometers follow the same general principle, but use weighted plungers, which usually result in greater displacements. Vertical movements of a plunger are correlated to values of IOP for particular applied forces. This is generally done using weights, as in the Schiotz Tonometer (available from, for example, Precision Optical Machine, Philadelphia Pa., USA), but it may also be done with springs. Alternatively, electronic indentation tonometers, such as the MacKay-Marg and the TonoPen (both available from Precision Optical Machine, Philadelphia, Pa., USA) have flat plungers that are sensitive to displacements of less than one micron.
Impression tonometers employ various weights with disks attached. The disks are inked with a dye and are used to indent the cornea. To measure the area of impression for different weights, the remaining dye is stamped onto a page and the diameter is measured. The Maklakov Tonometer (for example Barraquer 65/90-mm Hg Tonometer, Ocular Instruments, Inc., Bellevue, Wash., USA) is one example.
Non-contact tonometers have also been developed to avoid direct placement on the cornea. These tonometers cause a displacement either by a puff of air (U.S. Pat. No. 3,538,754), or by an acoustic beam (U.S. Pat. No. 5,865,742), rather than by mechanical means. The moment at which corneal flattening occurs may be sensed using a photoelectric means.
In all of these methods, there are many factors that contaminate the simple principle of IOP calculation, such as the following: (1) In flattening a corneal segment from a spherical surface to a plane, the volume of aqueous fluid under the dome is displaced and hence the IOP is increased. (2) Tear fluid fills the angle between the cornea and plane surface implying a larger corneal contact area than actually exists, which results in a lower IOP reading. (3) The surface tension of the tear meniscus adds another force to the applied force. (4) The cornea resists bending. (5) The corneal thickness and eye elasticity vary from individual to individual, which biases the pressure measurement. (6) The angle of force application is not measured, which biases the pressure measurement. (7) The corneal segment may be flat prior to the measurement due to an accident or medical intervention such as corneal sculpturing.
Regardless of the means of measuring the result of perturbing the cornea, the calculation method does not vary from system to system. Thus, inaccuracies and errors in calculation occur in all the existing devices.
Furthermore, the existing technology is not appropriate for a self-administered test. First and foremost, the present measurement devices measure directly on the cornea. Even non-contact tonometers require that the eye must be opened in order to perform the examination on the cornea, which, if performed improperly, may result in eye damage. Thus, the procedures cannot be safely administered at home.
There is provided, in accordance with an embodiment of the present invention, apparatus for measurement of intra-ocular pressure of an eye, including a distance-measuring unit and a processor. The distance-measuring unit measures a distance from an external surface of the eye to an internal element of the eye. The processor generates the intra-ocular pressure from at least the distance measurement.
There is provided, in accordance with another embodiment of the present invention, a non-invasive ocular pressure measuring unit, including a housing having a protuberance with a generally flat surface and a processor. The processor includes at least a force-measuring unit that measures a force applied by the protuberance to an eyelid.
There is provided, in accordance with another embodiment of the present invention, a method for detecting intra-ocular pressure of an eye, including the step of calculating a distance from an external surface of an eye to an internal element of the eye.
There is provided, in accordance with another embodiment of the present invention, a method for detecting intra-ocular pressure of an eye. The method includes placing an apparatus having a force-measuring unit against an eyelid, measuring a force resulting from the placement, and calculating an intra-ocular pressure from at least the force measurement.
There is provided, in accordance with another embodiment of the present invention, a method for correcting for material properties of an external element of an eye. The method includes the steps of measuring a force applied to the external element, measuring a distance from the external element to an internal element of the eye, and generating a value from a function relating the force and distance. The value relates to a zero displacement value of the function.
There is provided, in accordance with another embodiment of the present invention, a system for measurement of intra-ocular pressure of an eye. The system includes a housing with a protuberance with a generally flat surface and a processor. The processor includes a force measuring unit for measuring a force applied by the protuberance to an external surface of the eye, and a distance measuring unit for measuring a distance from the external surface of the eye to an internal element of the eye. The processor is configured to calculate a relationship between the force and distance measurements so as to produce a definable function of pressure over a range of values.